1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device in which a drive circuit is formed on a liquid crystal substrate.
2. Background Art
An active-matrix-type liquid crystal display device has been popularly used as a monitor of a personal computer, a television receiver set, an information display device of portable equipment or the like. The liquid crystal display device has the structure where a liquid crystal layer is sandwiched between a pair of substrates made of glass or the like on which pixel electrodes and counter electrodes are formed. By applying a voltage between the pixel electrodes and counter electrodes, the alignment direction of liquid crystal is changed. In this manner, by allowing the pixel electrodes and the counter electrodes to function as optical switching elements, an image is formed.
When the liquid crystal layer receives the application of the same voltage for a long time, the alignment direction of liquid crystal is fixed so that so-called burning occurs in the liquid crystal display device. To avoid this burning, in the liquid crystal display device, it is necessary to invert positive and negative polarities of a voltage applied to the liquid crystal layer for every fixed time, typically for every frame. Here, not only by alternately changing a voltage applied to the pixel electrode between two potentials consisting of a high potential and a low potential but also by alternately changing a voltage applied to the counter electrode between two potentials consisting of a high potential and a low potential, it is possible to decrease a width of the voltage applied to the pixel electrode thus reducing the power consumption.
As a method for changing a voltage applied to the counter electrode, several methods have been known. As such methods, a frame inversion method where voltages applied to all counter electrodes are set to the same potential, and the potential is changed for every frame, a line inversion method where a voltage having the same potential is applied to counter electrodes along a row (line) of pixels, and a voltage to be applied to the counter electrodes is changed for every row, a column inversion method where a voltage having the same potential is applied to counter electrodes along a column of pixels, and voltages to be applied to counter electrodes are changed for every column, a dot inversion method where voltages applied to counter electrodes of neighboring pixels are changed and the like are named. Among these methods, a line inversion method is superior to other methods in view of quality of an image display and easiness in forming a drive circuit.
JP-A-2006-276541 discloses a liquid crystal display device adopting a line inversion method where a counter electrode drive circuit is provided for every counter electrode signal line.
Further, in a so-called system-on-glass liquid crystal display device which forms a drive circuit per se on a liquid crystal substrate, an area which the drive circuit occupies on the substrate is decided depending on a scale of the drive circuit. In narrowing a picture frame of the liquid crystal display device or miniaturizing the liquid crystal display device, it may be possible to arrange a drive circuit on left and right sides of a display region in which pixels are formed in a two-split manner. By adopting such a constitution, a scale of the two-split circuit arranged on each lateral side of the display region is substantially halved compared to a case where the drive circuit is arranged on either one side of the display region.
JP-A-2004-61670 discloses a liquid crystal display device in which a gate driver circuit is arranged on left and right sides of a display region in a two-split manner (see FIG. 8 and the like).
FIG. 8A and FIG. 8B are views schematically showing a liquid crystal display device 1 which adopts a line inversion method and arranges a drive circuit on left and right sides of a display region 6 in a two-split manner. The display region 6 is a region for displaying an image and a plurality of pixels are formed in the display region 6. Here, assume that n-pieces of pixels are arranged in the longitudinal direction. On left and right sides of these pixels, a counter electrode signal drive circuits 3L, 3R which control voltages applied to counter electrode signal line portions CX1 to CXn are arranged respectively. The counter electrode signal line portions CX1 to CXn are made conductive with counter electrode portions of pixels within the display region 6, and voltages outputted from the counter electrode signal drive circuits 3L, 3R are applied to the respective counter electrode portions through the counter electrode signal line portions CX1 to CXn. As shown in the drawing, the odd-numbered counter electrode signal line portions CX1, CX3, . . . CXn−1 counted from an upper edge of the display region 6 are connected to the counter electrode signal drive circuit 3L arranged on the left side of the display region 6. On the other hand, the even-numbered counter electrode signal line portions CX2, CX4, . . . CXn counted from the upper edge of the display region are connected to the counter electrode signal drive circuit 3R arranged on the right side of the display region 6. In the drawing, symbol H or L with parenthesis which is affixed to an end of symbol CX1, CX2, . . . CXn indicative of the counter electrode signal line portion indicates a voltage applied to the counter electrode signal line portions CX1 to CXn, wherein H expresses the application of a high-potential voltage, and L expresses the application of a low-potential voltage.
FIG. 8A shows a state of the counter electrode signal line portions CX1 to CXn of the liquid crystal display device 1 in an odd-numbered frame. As can be clearly understood from the drawing, a high-potential voltage is applied to all odd-numbered counter electrode signal line portions CX1, CX3, . . . CXn−1 which are connected to the counter electrode signal drive circuit 3L, while a low-potential voltage is applied to all even-numbered counter electrode signal line portions CX2, CX4, . . . CXn which are connected to the counter electrode signal drive circuit 3R.
Further, FIG. 8B shows a state of the counter electrode signal line portions CX1 to CXn of the liquid crystal display device 1 in an even-numbered frame. In this case, a low-potential voltage is applied to all odd-numbered counter electrode signal line portions CX1, CX3, . . . CXn−1, and a high-potential voltage is applied to all even-numbered counter electrode signal line portions CX2, CX4, . . . CXn.
That is, as shown in these drawings, when the counter electrode signal line portions CX1 to CXn are simply connected such that the counter electrode signal line portions CX1 to CXn are connected to the left and right counter electrode signal drive circuits 3L, 3R alternately, the same potential is applied to all counter electrode signal line portions CX1 to CXn connected to the counter electrode signal drive circuit 3L, 3R on either left or right side.
In general, the counter electrode signal line portions CX1 to CXn are formed of a transparent conductive thin film such as an ITO (Indium Tin Oxide) thin film. However, such a transparent conductive thin film has relatively high resistance. Accordingly, although the counter electrode signal line portions CX1 to CXn exhibit a high voltage value at a position near the counter electrode signal drive circuits 3L, 3R to which the counter electrode signal line portions CX1 to CXn are connected, the larger a distance from the counter electrode signal drive circuits 3L, 3R, the more the voltage is lowered. This phenomenon is observed as lowering of brightness of pixels at positions remote from the counter electrode signal drive circuits 3L, 3R.
Further, in the line inversion method, a voltage applied to the counter electrode portions is changed for every frame. Here, being influenced by parasitic capacitance generated between a thin film transistor and the counter electrode portion arranged in a pixel, the characteristics of the thin film transistor is changed. Accordingly, a voltage value written by the thin film transistor differs between a case where a high-potential voltage is applied to the counter electrode portion and a case where a low-potential voltage is applied to the counter electrode portion. Assume that an n-MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is used as a thin film transistor, this phenomenon is observed as lowering of brightness of the pixel when the high-potential voltage is applied to the counter electrode portion and as the increase of brightness of the pixel when the low-potential voltage is applied to the counter electrode portion.
FIG. 9A to FIG. 9C are graphs showing the brightness distribution in the lateral direction of the pixels of the liquid crystal display device 1 shown in FIG. 8A and FIG. 8B.
In the drawing, a position Y in the lateral direction in the display region 6 of the liquid crystal display device 1 is taken on an axis of abscissas, and the brightness L of the pixel is taken on an axis of ordinates. Positions at left and right ends of the graph correspond to positions at left and right edges of the display region 6.
FIG. 9A shows the brightness distribution of the pixels in the liquid crystal display device 1 in an odd-numbered frame, and corresponds to FIG. 8A. In the drawing, a curve 20 indicates the brightness distribution of the pixel corresponding to the odd-numbered counter electrode signal line portions CX1, CX3, . . . CXn−1. In this case, a voltage is supplied to the counter electrode signal line portions CX1, CX3 . . . CXn−1 from the left counter electrode signal drive circuit 3L and hence, the brightness distribution is lowered rightward as shown in the drawing. Further, a high-potential voltage is supplied to the odd-numbered counter electrode signal line portions CX1, CX3, . . . CXn−1 and hence, the brightness as a whole is slightly lowered. To the contrary, with respect to a curve 21 which indicates the brightness distribution of the pixel corresponding to the even-numbered counter electrode signal line portions CX2, CX4, . . . CXn, a low-potential voltage is supplied to the counter electrode signal line portions CX2, CX4, . . . CXn from the right counter electrode signal drive circuit 3R. Accordingly, the brightness distribution is lowered leftward, and the brightness as a whole becomes slightly higher than the brightness indicated by the curve 20. A curve 22 indicated by a broken line in the drawing indicates the brightness distribution of the pixels over the whole display region 6 and is formed by synthesizing the curves 20 and 21. The curve 22 is slightly lowered leftward as shown in the drawing. This phenomenon implies that an image is slightly dark in the vicinity of the left edge of the display region 6.
FIG. 9B shows the brightness distribution in an even-numbered frame in the liquid crystal display device 1, and corresponds to FIG. 8B. In this case, the voltage value applied to the odd-numbered counter electrode signal line portions CX1, CX3, . . . CXn−1 and the voltage value applied to the even-numbered counter electrode signal line portions CX2, CX4, . . . CXn become opposite to the corresponding voltages values used in the odd-numbered frame and hence, the brightness of the pixels indicated by the curve 20 as a whole becomes slightly high, while the brightness of pixels indicated by the curve 21 as a whole becomes slightly low. As a result, a curve 23 which indicates the brightness distribution of the pixels over the whole display region 6 by a chained line becomes slightly lowered rightward. This phenomenon implies that an image is slightly dark in the vicinity of the right edge of the display region 6.
FIG. 9C is a graph which indicates the curve 22 in FIG. 9A and the curve 23 in FIG. 9B simultaneously. As shown in the graph, the brightness difference indicated by symbol 24 arises at edge portions of the display region 6. Since the curve 22 indicates the brightness distribution over the whole display region 6 in the odd-numbered frame and the curve 23 indicates the brightness distribution over the whole display region 6 in the even-numbered frame, eventually, in this liquid crystal display device 1, the brightness is changed by the brightness difference 24 at left and right edge portions of the display region 6 for every 1 frame. This phenomenon is observed as flickering at a screen edge.
The present invention has been made to overcome these drawbacks and it is an object of the present invention to reduce flicking at a screen edge in a liquid crystal display device in which a drive circuit is arranged on left and right sides of a display region in a two-split manner.